Systems for detecting physical movement in and forces on structures are known in the prior art to include generally, a plurality of transducers, a data collection network and a means to interpret, store and display the collected data.
Structures can include towers, vehicles, buildings, dams, railroad beds, airport runways, mining embankments, machinery and the like. Forces imposed upon the structures, temperatures and changes in dimensions of a structure are important for maintenance and for safety reasons.
There are many kinds of transducers which transform a physical measurement into an electrical response such as a force produced by an acceleration (accelerometer), dimensional change (strain gage), inclinational change (inclinometer), pH readings or a thermal change (thermometer). The electrical response from a transducer may be in the form of an analog electrical signal. Common analog electrical signals convey information proportional to a voltage, current, frequency, phase change, etc. A plurality of tranducers are mounted at selected locations of a structure. The usual practice is to mount the transducer at or close to the sites of the structure deemed to be most critical or to be most sensitive to changes, such as on the thinnest portion of a structural member to measure deformations, at the extremities of objects subject to rotation to measure acceleration and the like.
The usual practice is to conduct the analog electrical outputs or signals from each transducer to a centrally located means to interpret, store and display the data. The means to interpret, store and display is usually a data logger with a visual display and printer; although it has also been the practice to amplify the electrical outputs and to display the data on calibrated analog dials and/or charts, either individually or on a centrally located console. Another practice is to tie a computer with a video display and printer to a decentralized scanner network to interrogate the desired electrical output. In order to simplify the discussion, the word computer will be used to refer to the centrally located means to interrogate, interpret, store and display the data; although the means to interrogate, interpret, store and display the data can be any other type of equipment as mentioned above and may not need to be completely centralized.
Because the electrical outputs from the transducers must be transmitted over substantial distances, a high quality, shielded cable is desirable in order to minimize data loss, electrical distortions, and electromagnetic interference and noise. Even so, the distance over which such electrical signals may be transmitted is limited by conductor resistance and transmission losses. Because the present practice in many instances requires the analog outputs from each transducer to be individually wired to the computer or scanner unit, installing the data collection network can be expensive and time consuming. Often the cost of the data collection network is an appreciable cost in the monitoring system and usually the most susceptible to damage and changes. In an attempt to decrease cabling costs, multiplex systems have been used where the configuration of the network permits. To incorporate a multiplex, several transducers' outputs are inputted into a circuit which converts the analog data into a digital format and then time division multiplexes the various digital data along a common line to the computer, the computer program then separates and interprets the digital data according to the time the data is received. While multiplexing can be used on some systems, multiplexing cannot be used advantageously in all systems, particularly where the configurations of the transducers are so spaced that no net savings in cabling costs is obtained by multiplexing. In addition, because the number of data lines which can be multiplexed is limited by the capacity of the multiplex system used, it has been sought to develop a simplified data collection system having low cabling cost that conveys the information accurately to the computer and is expandable according to the user's needs. The system should be independent of the particular transducer used (i.e., strain gauge, accelerometer, inclinometer, thermometer, etc.). This means that the type of data transmitted by the network should also be uniform in nature and compatible with the computer to use advantageously the capacity of a readily available computer to interpret and reconstruct the information generated at each transducer.
Microelectronics in the form of integrated circuits substrated upon a chip have been recently undergoing significant technological developments. As increasing numbers of digitally based circuit elements are fabricated onto a silicon chip, the cost of a basic circuit function has decreased markedly as has the physical size of the electronics. The power required to operate these circuits on a chip has decreased to the point where the power required to run the electronics may in some instances be obtained from the energy contained in the digital input signals themselves; furthermore, the speed and reliability of these circuits have likewise increased. Unlike analog data, signals transmitted digitally are not degraded as long as each bit is correctly received. The noise or cross talk which would be objectionable in an analog signal have no degrading effect on digital signals, as long as the noise or cross talk is substantially weaker than the digital signals so that the presence of each binary bit can be distinguished and fully regenerated.
Should there be a chance of noise or cross talk near the same level, it has been the practice to use redundant error detection bits such as check sum or cyclic redundancy code (CRC) to verify correctness to digital transmission with a higher degree of certainty.
A system using sensor to computer interface modules is also known in which the analog output from sensors of various types is gathered and converted into digital form and transmitted upon request to a computer in ASCII format. This system does not provide for integration of microprocessor with the sensors.